There is an ongoing need in the photographic materials market to produce rapidly processable films and papers which demonstrate exceptionally high performance qualities. Rapid processing is made possible by the increase in the rate of development that comes from the use of silver chloride or silver chlorobromide emulsions of high chloride content. However, these emulsions have had characteristics that have not allowed the highest performance to be achieved. In particular there have been limitations in reciprocity and latent image keeping characteristics of these materials.
It is well known that silver halide emulsions can contain iridium ions and that the presence of iridium can have a significant effect on reciprocity failure. Reciprocity failure is the deviation from or non adherence to the reciprocity law which is described by the equation E=IXT where E is exposure, I is intensity of the illumination falling on the sensitive material, and T is the length of the exposure also called the exposure time. Iridium is beneficial in reducing deviations from the reciprocity law. The iridium effect is commonly described as reducing reciprocity failure in that it minimizes changes in the photographic response characteristics of a material when a change is made in the time of light exposure given to a silver halide emulsion.
A general summary of the use of iridium in silver halide emulsions is contained in B. H. Carroll, "Iridium Sensitization: A Literature Review", Photographic Science and Engineering, Vol. 24, No. 6, 1980. At the 1982 International Congress of Photographic Science at the University of Cambridge, R. S. Eachus presented a paper titled "The Mechanism of Ir.sup.+3 Sensitization in Silver Halide Materials".
Silver halide crystals which contain chloride and bromide ions and which have iridium added during their precipitation are also known to have a highly undesirable property of changing photographic performance as a function of the time that elapses between exposure and processing. A description of this behavior is contained in H. Zwicky, "On the Mechanism of the Sensitivity Increase With Iridium in Silver Halide Emulsions", The Journal of Photographic Science, Vol. 33, pp. 201-203, 1985. The behavior between the time of exposure and the time when processing starts is also called latent image keeping (LIK).
LIK change may be seen as either a loss in speed or density or a gain in speed or density depending on whether a blue, green, or red spectral sensitizing dye is present. In addition, changing the sensitizing dye can give a change in the magnitude or the LIK for a given reciprocity value.